The freeze-drying process generally comprises subjecting a frozen product to heating at reduced (subatmospheric) pressure to sublimate water contained therein and thereby dry the product.
The apparatus for carrying out this process can comprise a sublimation-type dryer and a condenser connected thereto, a dust separator being interposed between the sublimating-type dryer and the condenser.
In a known arrangement of this kind, the dust recovered from the separator and containing residual moisture is fed back by a worm conveyor to a lower stage of the dryer and is mixed with the granular product therein. This has the disadvantage of altering the appearance of granulate.
For example, in the freeze-drying of coffee, the return of the moist dust to the granulate in the dryer tends to make the coffee extract somewhat yellowish and to decrease the market value of the product without appreciably effecting the taste.
Moreover, the return of moist dust to the granulate in the dryer detrimentally affects the bulk density of the product.
These problems have not been solved heretofore without loss of a valuable fraction of the product.